How would a lack of water affect biotic factors in an ecosystem?
In a nutshell, basically everything would die or move away to a place where water is available.
Water is important to every organism. A great portion of chemical reactions that organisms use take place in water, so without water, organisms would not be able to metabolize anything. When you can't metabolize, you die.
Warm-blooded animals, especially those that are carnivorous, usually need a lot of energy, so they would be adversely affected if the region lacking water were large enough. Since usually carnivores will have a large area they hunt in, if they hunt, they might be able to relocate to a different area without being affected too adversely.
Plants would wilt and their cells would die in a process called plasmolysis. Primary consumers (herbivores) would suffer twofold as their prey (the plants) die and their water source disappeared. Scavengers might be okay for a little, living off of the water in dead organisms and taking advantage of the surplus of dead organisms they feed on.
Realistically, organisms would try to leave the area and go somewhere where there is more water, though. This is the primary strategy of migratory species. Some, however, would be incapable of leaving such as plants and fungi.